Conditions which can be measured (e.g. temperature, voltage, pressure, weight, distance, velocity, capacitance, etc.) are often digitized so that the measure can be expressed as discrete steps or digits. The first step in digitizing often includes the use of a transducer, a device that will convert energy from one form to another. For example, a piezoelectric crystal can be used to convert pressure variations into an analog voltage and the thermistor can be used to generate an analog voltage as a function of temperature. These voltages can then be digitized by an analog-to-digital convertor. Other transducers, however, can more directly generate a digital output. For example, an interferometer can convert a displacement into a changing optical interference pattern that can be converted into a pulsing, enhanced digital, voltage via a photocell.
A popular form of analog-to-digital converter utilizes a ladder of matched resistors to divide either the input voltage or a reference voltage into a series of levels and arrive at digitized or quantized representations of that voltage. When used in these devices, feedback is in the form of voltage.
Voltage-to-frequency converters have a frequency output which is continuously variable. To be more useful this output can be counted by a binary counter gated by a fixed clock or the output can be used to gate a fixed clock being used to drive a counter. Voltage-to-time converters such as integrators, can be used in a similar manner as a sustitute for the voltage-to-frequency converters. Another form of converter is the delta-modulator which generates a single weighed digital pulse train with the plurality of the pulses dependent upon a difference signal. This difference signal is created by subtracting the input voltage from the feedback voltage. The feedback voltage is generated by either a charge dispensing circuit or a digital-to-analog convertor.
Another related technique is described in U.S. Pat. No. 4,300,135 which discloses a system wherein an analog condition causes a change in the capacitor of a variable frequency oscillator (VFO). This analog condition changes the frequency of the VFO output which is applied to a phase-locked loop (PLL). The PLL discriminator has an error voltage output the magnitude of which is indicative of the frequency deviation of the oscillator, and therefore, indicative of the analog condition. This error voltage is analog and is not encoded except for detection of a voltage in excess of a set threshold.
U.S. Pat. No. 3,140,612 describes a system wherein a variable such as acceleration causes a displacement between a case and a mass suspended in that case. This displacement causes a change in the time lag between two ultrasonic signals used to measure the position of the mass. The output is a pulse whose width is a function of the position of the mass within the case and is therefore a function of an initial variable such as acceleration.
U.S. Pat. No. 3,294,958 describes a system wherein an analog voltage causes a change in the frequency of a voltage control oscillator (VCO). The VCO output is compared to a reference oscillator to generate two measures of the frequency differences between the oscillators. The first measure is fed back to linearize the VCO response to the analog voltage and the second measure is a difference frequency which is counted to provide a digital output representative of the analog voltage.
In U.S. Pat. No. 3,868,677 there is described a system wherein an analog voltage causes a change in the phase of a voltage control oscillator (VCO). The VCO output is compared to a referance oscillator with a phase detector. The phase detector output is a signal whose pulse width is a function of the analog voltage. The phase detector output gates a counter clocked by a reference oscillator. The phase detector output is also fed back negatively to form a phase-locked loop.
A form of analog-to-digital converter which represents a significant improvement over the analog-to-digital converters above described is disclosed in co-pending application filed by Robert G. Nelson on Dec. 28, 1983 bearing Ser. No. 566,314 and entitled "Methods and Apparatus for Analog-To-Digital Conversion" of which the present application is a continuation-inpart. The method described therein comprises the steps of generating two high frequency signals which are harmonically related and modifying the phase of one of the signals in response to an analog function. The phase of the signals is compared to determine the phase difference and an action taken to return the signals toward an in-phase state. A measure of the action taken is utilized as a measure of the instantaneous value of the analog function.
In one specific embodiment of an analog-to-digital converter described in the above mentioned application two high frequency oscillators are employed having digital outputs with the frequencies of the oscillators bearing a harmonic relationship one to the other. A representation of an analog condition is applied to cause a shift in phase in at least one of the digital signals. A comparator is then utilized to obtain a measure of the phase-shift and the measure of the phase-shift is employed to adjust the phase of the phase-shifted signal toward an in-phase condition. The measure of phase-shift results in the generation of a digital function representative of the amplitude of the adjustment utilized to return the phase-shifted signal towards an in-phase condition. The digital function is applied to a low-pass digital filter to produce a binary word representative of the instantaneous value of the analog condition. A preferred form of digital function is a single weighed digital function.
Yet another analog-to-digital converter is set forth in co-pending application of Robert G. Nelson and James D. Awtrey bearing Ser. No. 857,028, filed Apr. 29, 1986 and entitled "Analog-To-Digital Convertor" of which the present application is a continuation-in-part. That application describes an ana log-to-digital converter which includes a signal source having a fixed frequency output. Means including an oscillator comprised of a plurality of digital gates connected serially in a loop, has an output of pulses, harmonically related to the pulses from the source. At least one of the digital gates incudes impedance means responsive to an analog condition to form a modulator. Change in the value of the impedance changes the time constant in the modulator to effect a change in the pulses related to the instantaneous amplitude of the analog condition. The outputs of the source and of the means are applied to a comparator. Means responsive to the comparator are employed to correct the means pulses toward an in-phase state relative to the pulses of the source. A digital function is produced whose value is proportional to the correction made to the means pulses and is thus representative of the instantaneous value of the analog condition.